The field of the invention is optical transmission cables.
Underground cables have been located in the past with the use of cable tracers. However, cable tracers require a conductor to be a part of the cable. Such tracers cannot be used with totally dielectric cables. As used in this application, the word "dielectric" means a material that is substantially non-conductive of electricity.
Many customers, particularly in areas of heavy lightning incidence such as Florida, demand cables which are totally dielectric. This requirement minimizes the risk of lightning damage, but it requires other means to locate a buried cable.
Two patents assigned to Schonstadt Instrument Company disclose the use of permanent magnet identifiers in cables to aid in the location of buried cables; see U.S. Pat. Nos. 5,017,873, and 5,006,806, said patents being incorporated herein by reference. These patents and other literature discuss the use of magnetized ducts holding a cable and in strips wound about the cable in long spirals. The use of separate ducts is expensive, however, and the use of a slow spiral tape presents a problem if the cable is too flexible, since the spiral may lose its uniformity after the cable is spooled and unspooled. The use of ducts is not ideal in many cases due to space limitations or the additional costs of manufacturing a separate duct. Adding barium ferrite powder to the outer jacket melt is also undesirable, for mixing the powder at a low enough concentration to avoid interference with the jacket's desirable properties requires resort to an undesirable jacket thickness in order to have a magnetically detectable jacket. What is apparently needed is an alternative means of making a dielectric optical transmission cable.